Pressure-applying arrangement in a hydraulic axial piston machine

ABSTRACT

A pressure-applying arrangement in a hydraulic axial piston machine is disclosed, having a pressure plate (7) and a piston (9) that is axially displaceable in a cylinder body (3), is biased by a spring (10) and acts against the pressure plate (7). It should also be possible to use such a pressure-applying arrangement when the axial piston machine is operated with a hydraulic fluid that has no or only very little lubricating property, for example, water. For that purpose, the piston (9) is formed from a high-strength thermoplastics material.

BACKGROUND OF THE INVENTION

The invention relates to a pressure-applying arrangement in a hydraulicaxial piston machine, having a pressure plate and a piston that isaxially displaceable in a cylinder body, is biased by a force acting inan axial direction with respect to the cylinder body, and acts againstthe pressure plate.

By means of the pressure plate, slider shoes of work pistons are held inengagement with a slanting plate, which is inclined in known manner withrespect to the axis of the cylinder body, so that on rotation of thecylinder body the work piston is moved back and forth. Whereas theslider shoes have no problem engaging the slanting plate during theinward movement of the piston into the cylinder body, during the outwardmovement of the work piston they have to be held by the pressure plate.The pressure plate therefore always has to remain parallel to theslanting plate, so that as the cylinder body rotates, the pressure plateperforms a continuous tilting movement with respect to the cylinderbody.

To allow this tilting movement, U.S. Pat. No. 2,733,666 provides a ballbetween the pressure plate and the piston. The piston is here biased bya spring. Opening into the contact surfaces between the ball and thepiston and between the ball and the pressure plate there are channelsthrough which hydraulic fluid is able to penetrate to the contactsurfaces in order to reduce by lubrication the friction between the balland piston and between the ball and the pressure plate. Without suchlubrication, friction is relatively high so that this ball-and-socketjoint would wear very quickly. In an extreme case, it could even seizeup, leading to destruction of a part of the machine.

A hydraulic fluid that has a lubricating action is therefore anessential requirement here. This lubricating action is without exceptiona property of the hydraulic oils previously used as hydraulic fluids.Such oils are, however, in some cases toxic. From the point of view oftheir effect on the environment they are being used with increasingreluctance.

SUMMARY OF THE INVENTION

The problem on which the invention is based is to be able to use apressure-applying arrangement even when hydraulic fluids having littlelubricating action or even no lubricating action are to be used, forexample, water.

This problem is solved in a pressure-applying device of the kindmentioned in the introduction in that the piston is formed from ahigh-strength thermoplastic plastics material.

When using such a plastics material, the ball and the pressure plate cancontinue to be made of metal, as they were previously. Since, however,metal is no longer rubbing on metal but on plastics material,lubrication can largely be eliminated. In most cases, lubrication is notrequired at all. For the rest, a film of fluid, such as that provided bywater, for example, will be sufficient for lubrication.

The plastics material is preferably selected from the group of polyarylether ketones, especially polyether ether ketones, polyamides orpolyamide imides. Such plastics materials are particularly low-frictionin combination with metals, so that when they are used, furtherlubrication by means of oils or similar substances can be omittedwithout problems.

The plastics material is preferably reinforced by glass, graphite,polytetrafluoroethylene or carbon in fibre form. This measure enablesthe piston to be stressed by higher forces. Wear is reduced.

In a preferred construction, a ball is arranged between the piston andthe pressure plate. This is admittedly already known per se from U.S.Pat. No. 2,733,666. In combination with the plastics piston, however itsuse is even better, and also requires no lubrication.

Advantageously the piston has a diameter which is at least 30% largerthan the diameter of the ball. Because the plastics material, even whenit is high-strength plastics material, does not normally attain the samemechanical strength as a part made of steel or another metal, thissizing ensures that the piston is nevertheless able to transfer to thepressure plate the forces required for pressing the slider shoes againstthe slanting plate. The sizing prevents the piston from expanding as aresult of the counter-pressure exerted by the ball, leading to thepiston jamming in the cylinder body.

In this connection it is preferable for the ball to be inserted in anend-face recess of the piston having a depth that corresponds to 0.3 to0.4 times the diameter of the ball. The ball is therefore insertedrelatively deeply in the piston. This enlarges the contact surfacebetween the ball and the piston, but at the same time the surfaceloading is reduced, so that this measure enables improved coefficientsof friction to be achieved. In combination with the larger diameter ofthe piston, reliable guidance of the ball and a high mechanicalstability of the ball and piston arrangement is guaranteed.

In its furthest retracted position, the piston advantageously projectsfrom the cylinder body by a length that is larger than the depth of therecess. Even when the piston undergoes slight deformation as a result ofthe pressure acting on the piston, the piston cannot jam in the cylinderbody because the deformation is restricted to a region that alwaysremains outside the cylinder body.

This is achieved with great reliability in particular when the length isat least 40% greater than the depth of the recess. The length istherefore at least 1.4 times the depth of the recess. Any deformationsof the piston occurring in the region of the ball seat can continue fora short distance also in the axial direction, without the piston beingable to jam in the cylinder body.

The force acting in the axial direction on the piston is preferablygenerated by a spring which is guided in an axial bore in the piston andbears against the base of the piston, the piston base having a thicknessof at least 30% of the diameter of the ball. The piston base thereforehas sufficient mechanical strength for jamming of the piston in thecylinder body to be prevented quite easily. The piston base should be atleast as thick as the depth of the recess in which the ball is inserted.

In this connection is it especially preferable for the piston base to bethicker than the narrowest point of a circumferential wall radiallysurrounding the recess. Should deformation of the piston occur, thisdeformation is then effected in the region of the circumferential walland not at the piston base, so that an opportunity is provided fordeformations to become lost, as it were, which virtually excludesjamming of the piston in the cylinder body.

The pressure plate preferably has a recess receiving the ball, thecontact surface between the ball and the pressure plate being largerthan that between the ball and the piston. This ensures that the ballalways moves only relative to the piston and not relative to thepressure plate. Although the friction between ball and pressure plate isgreater in any case, because here metal rubs on metal, thecorrespondingly larger contact surface reinforces this effect even more.If the pressure plate moves relative to the cylinder body, the effect ofthis will always be that the ball slides only at the piston and does notrub on the pressure plate, so that wear and tear or destruction of theball by the pressure plate or of the pressure plate by the ball can beexcluded.

The pressure plate is preferably bevelled on its upper side facing theball, and is thinner at its radial edge than in the middle, and thisupper side, together with its opposite underside facing a slantingplate, forms an angle which is at least the same magnitude as the angleof inclination of the slanting plate. This ensures that the upper sideof the pressure plate does not conflict with the piston, even when thepiston projects relatively far in the direction of the pressure plate onaccount of the depth of its recess.

Advantageously, the pressure plate has at the lowest point of its recesssubstantially the same thickness as at the radial edge. This ensuresthat the pressure plate is able on the one hand to secure the ball withthe required reliability. On the other hand, the entire pressure plateneed not be dimensioned from the point of view of securing the ball. Inaddition, this construction enables a relatively uniform distribution offorces over the slider shoes.

It is also preferable for the piston, in particular in the region of itsrecess, to be in the form of a die-formed part. Since the metal ball isharder than the plastics material piston, larger tolerances thanpreviously can be accepted. Any variations in the spherical shape areevened out in operation by the pressure of the metal ball in theplastics material piston. Since the demands on tolerances are no longerso strict, it is possible to simplify the manufacturing process and inparticular to create the recess simply by die-forming.

In an especially preferred embodiment, on the upper side of the pressureplate there may even be formed a contact surface for the end face of thepiston. This has the advantage that rotation of the piston in thecylinder body, occasionally taking the form of a drifting movement, canbe avoided. The pressure plate rotates synchronously with the cylinderbody. When the pressure plate is in contact with the piston always atone point, the piston is held fixedly, which is sufficient to keep thepiston stationary in the cylinder body despite a possible disturbance bythe tilting movement of the pressure plate. Should such a movementnevertheless occur, it is harmless, that is, causes no further wear andtear, since the engagement of the piston with the pressure plate has aslittle friction as it has with the ball. This embodiment is especiallyadvantageous, however, because here the ball need not be used. Thepressure plate "rolls" on the end face of the piston, wherein thecontact surface can be described by a rotating radial ray. Since,however the piston and pressure plate are stationary with respect to oneanother in relation to the rotational movement, virtually no slidingfriction occurs at the end face of the piston.

The angle is preferably substantially the same magnitude as the angle ofinclination of the slanting plate. It is therefore not larger, but alsonot smaller, with the result that certain tolerances are allowed. Inthis manner the contact surface achieves its largest extent. Piston andpressure plate then lie adjacent to one another across the entire radiusof the end face of the piston. This allows a relatively uniformcompressive load per unit area. Wear and tear can therefore be avoided.

Advantageously, the contact surface extends right up to bores which areprovided in the pressure plate for receiving slider shoes. The smallestdistance between the contact surface and such a bore is here at most 25%of the radius of the piston. By this means, the contact surface can beselected to be as large as possible without the function or the mobilityof the slider shoes being in any way impaired.

The invention is described hereinafter with reference to a preferredembodiment in conjunction with the drawing, in which

FIG. 1 shows a diagrammatic cross-section through a hydraulic axialpiston machine,

FIG. 2 shows an enlarged fragmentary view from FIG. 1 and

FIG. 3 shows an enlarged fragmentary view from a second embodiment.

A hydraulic axial piston machine 1 has a cylinder drum 3 rotatablymounted in a housing 2. Work pistons 4 are mounted in the cylinder drum3 so as to move in an axial direction. Each work piston 4 is guidedduring this movement by a slider shoe 5 on a slanting plate 6. Theslider shoe 5 is held in engagement with the slanting plate 6 by apressure plate 7. Via the intermediary of a ball 8, the pressure plate 7engages a piston 9 housed in the cylinder drum 3. The piston 9 is biasedby a spring 10 acting in the axial direction, that is to say, it ispressed towards the slanting plate 6.

As is generally well known, on rotation of the cylinder drum 3 the workpistons 4 are moved back and forth in an axial direction. Since thepressure plate 7 must always remain parallel to the slanting plate 6, itperforms a continuous tilting movement with respect to the cylinder drum3. Here, the ball 8 represents an articulated joint between the pressureplate 7 and the cylinder drum 3. Relatively small axial movements of thecylinder drum 3 are compensated for by the spring 10, that is to say,even when the cylinder drum 3 undergoes relatively small axial movementsthe pressure plate 7 remains biased in such a way that the slider shoes5 are always held in engagement with the slanting plate 6.

The machine 1 is intended to be operated with water as the hydraulicfluid. For that purpose the pressure-applying arrangement, which isconstituted essentially by the pressure plate 7, the ball 8, the piston9 and the spring 10, is designed so that it can also operate withoutlubrication by the hydraulic fluid. This is achieved in that the piston9 is formed by a high-strength thermoplastic plastics material, which isselected from the group of polyaryl ether ketones, especially polyetherether ketones, polyamides or polyamide imides. The plastics material isreinforced by glass, graphite, polytetrafluoroethylene or carbon, thisreinforcement being in the form of fibres. The ball 8 and the pressureplate 7 can still be made of metal. The ball 8 is accordingly in mostcases harder than the piston 9. If a force is exerted by way of thepiston 9 on the pressure plate 7, there is a danger that the piston willbe deformed. Such a deformation will not be noticeable in most cases.If, however, the piston 9 is housed in the cylinder drum 3 with arelatively small tolerance, such a deformation could lead to jamming.Moreover, regardless of the choice of material, the piston 9 must, ofcourse, be capable of transmitting the forces acting on the pressureplate 7.

For that purpose, the piston 9 first of all has a diameter D2 which isat least 30% larger than the diameter D1 of the ball 8. This enables theball 8 to be accommodated in a end-face recess 11 of the piston 9 whichhas a relatively large depth a. This depth corresponds to 0.3 to 0.4times the diameter D1 of the ball 8. A relatively large proportion ofthe ball 8 is therefore surrounded by the piston 9. The ball 8 isconsequently guided in the piston 9, even laterally, in a very stablemanner.

At its end opposite the ball 8, the piston 9 has a clearance space 12 ofa length d in which to move, that is to say, it can be retracted furtherinto the cylinder drum 3 by the distance d. When the piston 9 isretracted as far as it will go into the cylinder drum 3, it stillprojects by a length 1 with its ball end. In the position illustrated,in which the piston 9 is not retracted into the cylinder drum 3 as faras it will go, the length d of the clearance space 12 is added to thislength 1. At all events, the length 1 is calculated so that it isgreater than the depth a of the recess 11. It should be at least 40%greater than the depth a of the recess 11, so that deformations that maypossibly occur because of force exerted by the ball 8 do not lead to thepiston 9 jamming in the cylinder drum 3. The deformations are thenrestricted to a region that at any rate still projects from the cylinderdrum 3.

The spring 10 is guided in the piston 9 in an axial bore 13 and bears ona piston base 14. The piston base has a thickness b which is at least aslarge as 30% of the diameter D1 of the ball 8. The thickness b of thepiston base 14 is at any rate larger than the thinnest point of acircumferential wall 15 radially surrounding the recess 11. Deformationswill then occur in the circumferential wall 15 rather than in the pistonbase 14. The thickness of the circumferential wall is determined by thedifference in the diameters D1 and D2 of the ball 8 and the piston 9divided by two.

The pressure plate has a recess 16 receiving the ball 8, in which theball 8 is inserted to about half-way. The contact surface between theball 8 and the pressure plate 7 is therefore larger than that betweenthe ball 8 and the piston 9. The friction between the ball 8 and thepressure plate 7, which is in any case greater, on account of themetal-to-metal material combination, than between the ball 8 and thepiston 9, is further increased by the larger contact surface, so thatwhen the pressure plate 7 moves with respect to the piston 9 the ball 8will rotate in the piston 9 but not, however, in the pressure plate 7.

The pressure plate 7 is bevelled on its upper side facing towards thepiston 9; at its radial edge it is thinner than in its middle. With theopposing underside 18 the upper side 17 forms an angle α2 (the angleillustrated is the corresponding counter-angle of the same magnitude),which is at least the same magnitude as the angle of inclination α1 ofthe slanting plate 6. Although a relatively large proportion of the ball8 is surrounded by the piston 9, conflict or interference between thepressure plate 7 and the piston 9 can consequently be reliably avoided.It is even possible to provide a contact surface 19 between the piston 9and the pressure plate 7, although this is normally avoided by matchingthe depth of the recesses 11 and 16 suitably to the diameter of the ball8.

The pressure plate 7 has a thickness h1 at the deepest point of itsrecess 16 which is essentially the same as the thickness h2 at itsradial edge. This thickness determines the minimum stability of thepressure plate 7. By bevelling the upper side 17, however, the forceintroduced by way of the piston 9 and the ball 8 onto the pressure plate7 is able to spread itself relatively uniformly from the inside to theoutside, which results in flush engagement of the slider shoe 5 on theslanting plate 6.

An advantage of the pressure-applying arrangement is that onlyrelatively modest demands are made on tolerance during manufacturebecause the harder ball 8 will in operation gradually even outrelatively small variations in the recess 11 of the piston 9. Because ofthe low demands on tolerance, the piston 9 can be manufactured as adie-formed part. The recess 11 at least can be produced by die-forming,which is a relatively inexpensive manufacturing method, without thefunction of the pressure-applying arrangement being adversely affected.

FIG. 3 shows an enlarged fragmentary view from a second embodiment of apressure-applying arrangement, which functions even without a ballbetween the piston 9' and the pressure plate 7'. Identical parts areprovided with the same reference numbers and corresponding parts areprovided with dashed reference numbers. If the ball is omitted, only theshape of the pressure plate 7' and the shape of the piston 9' alter. Thecontact surface 19' is enlarged correspondingly in a radially inwarddirection. The contact surface 19' can be described by a radial raywhich starts at the centre point of the end face of the piston 9' andextends to the edge. The contact surface 19' will, of course, be given acertain width owing to the material characteristics. When the pressureplate 7' moves with respect to the slanting plate 6, the contact surface19' rotates about the centre point of the end face of the piston 9'.There is therefore a kind of rolling movement between the pressure plate7' and the piston 9', wherein sliding of the two parts against oneanother can be largely avoided. The frictional losses can here be keptvery low specifically by the geometrical construction of the piston 9'and the pressure plate 7'. They are additionally reduced in that thepiston 9' consists of the above-mentioned plastics material, inparticular from the group of polyether ether ketones.

The contact surface 19' extends right up to bores 20 which are providedfor receiving the slider shoes 5 in the pressure plate 7'. The contactsurface between the piston 9' and the pressure plate 7', as far as itgoes, is by that means enlarged and the compressive load per unit areais correspondingly reduced. The distance to the bores 20 is, on theother hand, still large enough for the function and the mobility of theslider shoes 5 in the pressure plate 7' not to be hindered. Theadditional length, that is to say, the distance between the piston 9'and the bores 20, should at its smallest point be about 10 to 20%, atany rate not more than 25%, of the radius of the end face of the piston9'. In this way the pressure plate 7' also is relatively uniformlystressed. This has an advantageous effect on the tilting behaviour ofthe slider shoes 5.

I claim:
 1. A pressure-applying arrangement in a hydraulic axial pistonmachine, the machine having a pressure plate and a piston, said pistonbeing axially displaceable in a cylinder body, being biased by a forceacting in an axial direction with respect to the cylinder body, andacting against the pressure plate, the piston being formed from ahigh-strength thermoplastic plastic material;a ball is arranged betweenthe piston and the pressure plate; said piston has a diameter which isat least 30% larger than the diameter of the ball; and the ball isinserted in an end-face recess of the piston having a depth thatcorresponds to 0.3 to 0.4 times the diameter of the ball.
 2. Anarrangement according to claim 1, in which, in its furthest retractedposition, the piston projects from the cylinder body by a length that islarger than the depth of the recess.
 3. An arrangement according toclaim 2, in which the length is at least 40% greater than the depth ofthe recess.
 4. An arrangement according to claim 1, in which the forceacting in the axial direction on the piston is generated by a springwhich is guided in an axial bore in the piston and bears against a baseof the piston, the piston base having a thickness of at least 30% of thediameter of the ball.
 5. An arrangement according to claim 4, in whichthe piston base is thicker than a narrowest point of a circumferentialwall of the base radially surrounding the recess.
 6. An arrangementaccording to claim 1, in which the pressure plate has a recess receivingthe ball, a contact surface in the pressure plate recess between theball and the pressure plate being larger than a contact surface betweenthe ball and the piston.
 7. An arrangement according to claim 6, inwhich the pressure plate has a thickness at a deepest point of saidrecess which is essentially the same as a thickness at a radial edge ofsaid recess.
 8. A pressure-applying arrangement in a hydraulic axialpiston machine, the machine having a pressure plate and a piston, saidpiston being axially displaceable in a cylinder body, being biased by aforce acting in an axial direction with respect to the cylinder body,and acting against the pressure plate, the piston being formed from ahigh-strength thermoplastic plastic material in which the pressure plateis bevelled on an upper side facing the ball, and is thinner at itsradial edge than in its middle, and said upper side, together with itsopposite underside facing a slanting plate, forms an angle which is atleast as large as an angle of inclination of the slanting plate.
 9. Apressure-applying arrangement in a hydraulic axial piston machine, themachine having a pressure plate and a piston, said piston being axiallydisplaceable in a cylinder body, being biased by a force acting in anaxial direction with respect to the cylinder body, and acting againstthe pressure plate, the piston being formed from a high-strengththermoplastic plastic material, in which on an upper side of thepressure plate there is formed a rotating contact surface facing thepiston, and in which the contact surface extends up to bores which areprovided in the pressure plate for receiving slider shoes.